1. Field of the Invention
This invention relates to a diffraction grating for use in a spectroscopic instrument, optical communication instrument or the like and a method of manufacturing the same.
2. Description of the Prior Art
Diffraction gratings have been mainly used in optical measuring instrument as a wavelength dispersion element for selectively extracting a preferable wavelength from a certain waveform region. However, in general, the diffraction grating has a diffraction efficiency, which is largely affected by the wavelength, incident angle and polarization of the incident light. However, if conditions of use are limited, it is possible to optimize the sectional profile of the grating to obtain a high performance diffraction grating. At present, research is being conducted in this area.
For example, an article in Appl. Phys. 24, No. 2 pp. 147-150 (1981) shows an experimental verification in the microwave region, of the idea that based on the fact that the manufacture of Fourior gratings by composing their groove profile is possible by combining a fundamental harmonic and the second order harmonic through the holographic exposure technology, the efficiency of Fourior gratings can be made higher than that of Echelette gratings for large wavelength region applications by locally optimizing the groove profile of the grating, in the microwave region.
Here, the Fourior gratings generally have a groove profile .eta. (x) which can be expressed, with the direction perpendicular to the groove direction taken as the x-axis and the groove pitch expressed as d, in terms of the fundamental harmonic K and the second order harmonic 2 K, where K=2.pi./d, as follows: EQU .eta.(x)=h[sin (Kx)+.gamma.sin (2Kx+.phi.)]
where h, .gamma. and .phi. are parameters for determining the profile. The groove profile of diffraction gratings described in the above-mentioned article corresponds to the case when h=0.42, .gamma.=0.286 and .phi.=-90.degree..
In the article mentioned above, however, the local optimization is made under the condition that the angular deviation between the light incident to the diffraction grating and the first order diffracted light reflected therefrom is 17.degree.. This means that, for example, light is incident thereto at .lambda.d=1 from an angle different by 8.5.degree. from the Littrow angle.
When diffraction gratings are employed for optical communication applications, they are used in the neighborhood of the Littrow mount in many cases, and there is no case in which they are used while maintaining an angular deviation of 17.degree.. Therefore, the superiority of the Fourior gratings in the local optimization for the Littrow mount is not described quantitatively in the above-mentioned article. This is the first problem to be pointed out. Also, although a theoretical prediction has been verified experimentally in the microwave region, the groove profile of diffraction gratings actually experimented with is considerably different from that which is locally optimized.
In general, optical communication uses a light having a wavelength range from 0.8 .mu.m to 1.55 .mu.m which is considerably shorter than that of the microwave range. As a result, it cannot be concluded that the superiority of a diffraction grating locally optimized could be verified for all wavelength regions by having it verified only in the microwave range. This is the second problem to be pointed out.
In addition, the groove profile obtained by the calculation in the above-mentioned article is only appropriate for a perfectly conductive groove surface. Therefore, the theoretically calculated profile is not valid for a groove surface which is an imperfect conductor.
As to polarization, an article in J. Opt. Soc. Am. A, 3, No. 11 pp. 1780-1787 j(1986) describes that if the light waves polarized perpendicular and parallel to the groove direction, indicated as a TM wave and a TE wave, respectively, the groove profile is rectangular, but due to the conductivity that a metal of the surface of the diffraction grating has, variation of the efficiency of the TM wave due to the incident angle of a light is more rapid than that of the TE wave particularly in a wavelength range of several micro or less meters as compared to the situation where the reflecting surface is perfectly conductive. This is the third problem to be pointed out.
The conventional manufacturing method used in the two-beam interference exposure method, using a coherent light, but the photosensitive characteristic of a photosensitive layer to be formed on a substrate is produced in the range in which the film thickness residue of the photosensitive layer after development is approximately linearly related with the exposure time, so that in actually making Fourior gratings, for example, as in Optica Acta 26, No. 11, pp. 1427-1441 (1979), two exposure processes and precision positioning control are required.
In addition, in the holographic exposure technology, it is the general practice to expose an interference pattern obtained by superimposing plane waves with respect. In this case, however, since the coherent light from a spacial filter is a divergent beam, the spacial coherence will be disturbed by optical elements such as, for example, lenses and mirrors which are disposed between the substrate and the spacial filter in order to change a divergent wave into a plane wave, causing a slight distortion of the interference pattern. To avoid such a problem, a complicated optical system is required.